Conducting polymers have been used in organic electronic devices for many years. They have played a role in forming conductive tracks and interconnections in organic thin film transistors, forming the source, drain and gate electrodes in these devices. In addition, they have also been implemented in organic light emitting diodes as a hole injection material in order to improve the charge carrier injection into the light-emissive material from a non-organic anode such as indium tin oxide (ITO).
Organic, particularly polymeric electronic devices are extremely attractive from a manufacturing aspect due to the possibility to fabricate devices in ambient conditions.
Functional electronic applications must comprise multiple devices. For example, a “smart card” may have a display, a display driver and a solar cell for producing power. In addition, a memory chip for data storage is also of importance to fabricate fully functional devices.
In inorganic devices, ceramic materials are utilised in order to achieve retention of data, thus allowing fabrication of memory chips. In the organic, or polymer field, ferroelectric polymers such as those based on poly(vinylidene fluoride) (PVDF) may be used to fabricate these devices. A particular example of a PVDF based polymer in which a memory effect has been demonstrated is poly(vinylidene fluoride-trifluoroethylene) (P(VDF/TrFE)). The testing of such materials for their applicability in a memory device can be performed by the fabrication of a cross point device. Cross point devices are based on the same principles as thin film capacitors.
An example of a cross point device is shown in FIG. 1a. Two electrodes 10, 11 are applied either side of a thin ferroelectric film 20, typically in the range of 200 nm to 2 microns in thickness, using metallic materials. Upon application of an electric field applied between the electrodes 10, 11, a polarisation response can be measured as a function of the electric field. A hysteretic nature in the polarisation vs. field plot will indicate the suitability of the material for a memory device.
An example of a ferroelectric memory is shown in FIG. 1b, in which a plurality of rows of electrodes 10a are provided under the ferroelectric film 20 and a plurality of columns of electrodes 10b are provided above the ferroelectric film. In a manner well-known in the art, the rows and columns of electrodes can be addressed to polarise the ferroelectric material at the intersection between an addressed row and an addressed column, thereby writing data. This data can subsequently be read by determining the polarisation of the ferroelectric material at the intersection between the addressed row and column.
More specifically, at each cross point, the top and bottom electrodes form a “bit” in a memory device, and can be read as a “1” or a “0” according to the spontaneous polarisation of the ferroelectric material. The spontaneous polarisation of a ferroelectric material is given by the value of the dipole moment per unit volume of material. In a ferroelectric material, the direction of the spontaneous polarisation can be switched by the electric field, and hence a polarisation hysteresis can be measured.
However, the hysteretic properties of the material are not the only requirements in order to be suitable for use in a ferroelectric polymer device. The compatibility of the polymer material in an existing fabrication framework of processes must also be addressed.
Fluorinated polymers such as that mentioned above may be used in ferroelectric capacitors, and are soluble in polar solvents such as 2-butanone. The high electronegativity of the fluorine atoms in the structure gives rise to this high polarity of the material, and therefore solubility in such a solvent. In addition to a high polarity, the fluorine content in these polymers also gives rise to a strong hydrophobic nature.
The contact angle of a water droplet on the surface of a thin film of this type of polymer is equal to or greater than 90 degrees. By exhibiting such a high contact angle, it is problematic to deposit or print a water based dispersion or solution of material on such a surface.
PEDOT:PSS is widely used as a conducting polymer in many organic devices, as explained above. PEDOT:PSS is widely and commercially available, for example in the form of Baytron-P solution, produced by H C Starck. The commercial Baytron-P solution is a water-borne solution of poly(ethylene dioxylthiophene) (PEDOT) in the presence of poly(styrene sulphonic acid) (PSS), which serves as a colloid stabiliser and dopant. Thus, the material is a dispersion of particles (in the nanometre scale) based in water and, consequently, when this material is deposited on the surface of a PVDF (or a co-polymer) film, the same de-wetting behaviour is exhibited.
This problem has been previously recognised and is addressed in WO 02/43071. Specifically, WO 02/43071 discloses a ferroelectric memory circuit comprising a ferroelectric memory cell in the form of a ferroelectric polymer thin film and first and second electrodes on either side. The electrodes are conducting polymer electrodes which are deposited on top of a ferroelectric thin film by spin coating from an H C Starck Baytron-P solution or dipping in such a solution. WO 02/43071 discloses that in the case of spin coating a certain amount of surfactant must be added to the Baytron-P solution to allow a uniform and smooth PEDOT/PSS film formation. However, neither the amount nor the nature of the surfactant to be added to the spin coating solution is disclosed in WO 02/43071.
WO 2005/064705 also discloses a ferroelectric device in which an aqueous PEDOT:PSS solution is deposited by spin coating on a ferroelectric polymer layer. To overcome the de-wetting properties of ferroelectric polymer layer, n-butanol is added in the aqueous solution as a surface-tension reducing agent with a concentration of 3% or lower, so that the solution remains in a single phase. In addition, a cross-linking agent may be provided in the aqueous solution.
Hitherto, and in both WO 02/43071 and WO 2005/064705, the PEDOT:PSS is deposited on the ferroelectric layer by spin coating or dipping so that it covers the whole surface of the ferroelectric layer. Subsequently, the PEDOT:PSS layer is patterned using known techniques, such as photolithography.
However, the use of such patterning techniques is undesirable as they require the highly accurate alignment of a mask over the layer to be patterned. Where it becomes necessary to pattern several layers, which is common in the formation of electronic devices or circuits, such as transistors and ferroelectric devices, difficulties with alignment are increased. Thus, the speed of manufacture is reduced and the cost of the devices is increased.
An attractive aspect of forming electronic devices using organic materials is the possibility of using flexible substrates, and reel-to-reel processing. However, the difficulties in the alignment required by patterning techniques are exacerbated and become prohibitive.
It is known to deposit conductive polymers on substrates using ink jet techniques, where the solution in which the conductive polymer is deposited has good wetting properties with respect to the substrate on which it is deposited. However, the de-wetting properties of fluorinated ferroelectric polymers with respect to aqueous solutions of conductive polymer have made it impossible to form a continuous conducting path of PEDOT:PSS by ink jet printing. The de-wetting nature in the drying process means that a “shrinking” type droplet is formed upon deposition. Such shrinking means that bridging of adjacent droplets with a high uniformity is not possible.
To illustrate, FIG. 2 shows the chemical structure of PEDOT:PSS. In a typical aqueous dispersion such as the commercially available Baytron-P solution suitable for ink jet printing on a hydrophilic surface, the solid content is in the region of 0.3 to 1% by volume, corresponding to around 6 to 15 cP in viscosity at room temperature. However, FIG. 2 shows the pattern that results when droplets 30 of such an aqueous solution are ink jet printed on a fluorinated polymer layer 20. Although the droplets are of the same size and are equidistantly spaced when deposited by ink jet printing so that a continuous track would be formed were there to be no de-wetting properties, in practice the droplets 30 shrink to different sizes and fail to form a continuous conducting path. More specifically, the nature of the de-wetting process means that during the shrinking of a droplet on the surface, a lateral movement of this droplet may occur, resulting in an irregular pattern being formed. In addition to this lateral movement, droplets may coalesce in order to minimise the surface area before drying. Thus, a dried printed line may contain differing sizes of droplets on the surface. This phenomenon also hinders the formation of continuous tracks of inks.
The suitability of the prior art aqueous solutions of PEDOT:PSS and surface tension-reducing agents for ink jet printing on hydrophobic surfaces has not been demonstrated. In particular, solutions that are suitable for spin coating are not adapted or suitable for use with ink jet heads. Consequently, the formation of conducting polymers on hydrophobic surfaces using ink jet printing techniques has not hitherto been considered as workable.